Perceptive or not, it's not an issue with the GPL. Someone can come back to you with proprietary code inside the same BSD licensed project. Or proprietary inside a GPL. Or AGPL inside GPL. GPLv2 inside GPLv3.

You make assumptions when you use code you didn't write. It's as simple as that. You assume that the distribution of the code is proper, and that the license is as described, and that everyone who contributed did so honestly.

Proprietary or open, it's a problem everywhere, and not limited to just the GPL.